A fuel cell discharges small amounts of harmful gases and has a high efficiency of power generation, and it is thus expected that a fuel cell is applied to a wide variety of generation systems such as large-scale power generation, a cogeneration system, an automobile power source, and the like. Particularly, a solid-oxide fuel cell (referred to as a solid-electrolyte fuel cell) operates at 700° C. to 1000° C., and has the characteristics that a catalyst need not be used for an electrode reaction, various fuel gases such as a coal reformed gas, and the like can be used, the fuel cell can be combined with a gas turbine or steam turbine by using high-temperature exhaust heat. Therefore, fuel cells attract attention as an energy source for the next generation.
As shown in FIG. 1, an example of the solid-oxide fuel cells comprises an electrolyte 1, electrodes 2 and 3, and interconnects 4 (referred to as “separators”), the electrolyte 1 generally comprising an ionic conductive solid electrolyte such as yttria stabilized zirconia (YSZ) or the like. The cathode (air-electrode) 2 composed of (La,Sr)MnO3, or the like and the anode (fuel-electrode) 3 composed of Ni/YSZ (cermet of Ni/yttria stabilized zirconia (YSZ)) or the like are attached to both surfaces of the electrolyte 1 so that the electrolyte 1 functions as a partition, a fuel gas 5 such as hydrogen gas is supplied to one of both sides of the electrolyte 1, and an oxidizing gas 6 such as air or the like is supplied to the other side to take out electricity. The interconnects 4, comprising 3 layers of the electrolyte 1 and the electrodes 2 and 3, have the function to support an electrolyte-electrode assembly, form gas channels 7 and pass a current.
At present, the solid-oxide fuel cells have many problems of practical use remaining unsolved. Particularly, the interconnects 4, which are important components, have many problems. This is because the interconnects are used at a high temperature near 1000° C., for example, 700° C. to 900° C., and thus the interconnects are required to have properties such as oxidation resistance, electrical conductivity, and a small difference in thermal expansion from the electrolyte.
As a material satisfying these requirements, conductive ceramics such as (La,Sr)CrO3, and the like are conventionally used. However, ceramics have low workability and are expensive, and thus ceramics have problems from the viewpoint of increase in size and practical use of fuel cells. Therefore, the development of an interconnect has been progressed by using an inexpensive metal material having high reliability as an alternative material.
In the use of a metallic material at a high temperature, the surface is oxidized to form an oxide layer thereon. Therefore, in order to use such a metallic material for interconnects, it is necessary that the oxide layer slowly grows and does not peel, and the oxide layer has electrical conductivity. Namely, it is necessary that the oxide layer has both the high-temperature oxidation resistance and electrical conductivity.
As a technique for satisfying these requirements, for example, Japanese Unexamined Patent Application Publication No. 6-264193 discloses austenitic stainless steel used as a metallic material for solid-oxide fuel cells, the austenitic stainless steel comprising 0.1 percent by mass or less of C, 0.5 to 3.0 percent by mass of Si, 3.0 percent by mass or less of Mn, 15 to 30 percent by mass of Cr, 20 to 60 percent by mass of Ni, 2.5 to 5.5 percent by mass of Al, and the balance substantially composed of Fe. However, the metallic material contains significant amounts of Al and Cr, and thus forms an oxide layer mainly composed of an Al oxide. As described below, an Al oxide has low electrical conductivity, and is thus unsuitable for use for interconnects of solid-oxide fuel cells. Since the austenitic stainless steel has a high thermal expansion coefficient (a thermal expansion coefficient of 16 to 20×10−6/° C. from 20° C. to 900° C.), as compared with that (a thermal expansion coefficient of 9 to 12×10−6/° C. from 20° C. to 900° C.) of the yttria stabilized zirconia of the electrolyte 1, the electrolyte or the electrodes are possibly cracked due to a difference in thermal expansion when the temperature changes at a start or stop. Also, 20 to 60 percent by mass of expensive Ni must be added.
Also, Japanese Unexamined Patent Application Publication No. 7-166301 discloses a technique for interconnects of solid-oxide fuel cells, in which an element (La, Y, Ce, or Al) is added to a material comprising 60 to 82 percent by mass of Fe and 18 to 40 percent by mass of Cr, for decreasing the contact resistance between an interconnect and an air-electrode (cathode) of an electrical cell. However, the interconnect material does not have oxidation resistance such as resistance to long-term use at high temperatures, thereby inevitably causing the problem of increasing the electrical resistance of the oxide layer.
Furthermore, Japanese Unexamined Patent Application Publication No. 7-145454 discloses a metallic material for solid-oxide fuel cells, the material comprising 5 to 30 percent by mass of Cr, 3 to 45 percent by mass of Co, 1 percent by mass or less of La, and the balance substantially composed of Fe. However, the material does not have a sufficient property from the viewpoint of oxidation resistance, particularly, a weight increase by oxidation.
Furthermore, Japanese Unexamined Patent Application Publication No. 9-157801 discloses a steel material for interconnects of solid-oxide fuel cells, the material comprising 0.2 percent by mass or less of C, 0.2 to 3.0 percent by mass of Si, 0.2 to 1.0 percent by mass of Mn, 15 to 30 percent by mass of Cr, 0.5 percent by mass or less of Y, 0.2 percent by mass or less of REM, 1 percent by mass or less of Zr, and the balance substantially composed of Fe. The amount of descaling of this material is evaluated for oxidation resistance. However, an increase in thickness of the oxide layer is not sufficiently inhibited, and thus the electrical resistance is inevitably increased by growth of the oxide layer. Also, the thermal expansion coefficient is not sufficiently decreased.
Furthermore, Japanese Unexamined Patent Application Publication No. 10-280103 discloses a steel material for interconnects of solid-oxide fuel cells, the material comprising 0.2 percent by mass or less of C, 3.0 percent by mass or less of Si, 1.0 percent by mass or less of Mn, 15 to 30 percent by mass of Cr, 0.5 percent by mass or less of Hf, and the balance substantially composed of Fe. Like the material disclosed in Japanese Unexamined Patent Application Publication No. 9-157801, the amount of descaling of this material is evaluated for oxidation resistance. However, an increase in thickness of the oxide layer is not sufficiently inhibited, and thus the electrical resistance is inevitably increased by growth of the oxide layer. Also, the thermal expansion coefficient is not sufficiently decreased.
As described above, any one of the conventional disclosed metallic materials does not necessarily have sufficient oxidation resistance and electrical conductivity for interconnects of solid-oxide fuel cells.
In a metallic material for interconnects of solid-oxide fuel cells used in an environment of 700° C. to 1000° C., particularly 700° C. to 900° C., a protective oxide layer must be formed for maintaining the oxidation resistance. However, an interconnect is a member required to have electrical conductivity, and thus the oxide layer must have electrical conductivity and must be thinned.
However, an Al oxide which forms an excellent protective layer has low electrical conductivity, and thus the formation of the oxide layer significantly deteriorates the performance of a cell due to an increase in the electrical resistance. Therefore, a material containing a large amount of Al cannot be used for interconnects. Even when a Fe—Cr alloy is used to form a Cr oxide layer having high electrical conductivity, the alloy cannot be used for interconnects unless the adhesion and the growth rate of the layer can be decreased. Namely, a Fe—Cr alloy to which only REM is added cannot be sufficiently used for interconnects.
It could therefore be advantageous to provide an inexpensive metallic material (Fe—Cr alloy) for interconnects of solid-oxide fuel cell, and a fuel cell using the metallic material, the metallic material having an excellent oxidation resistance at a high temperature of 700° C. to 900° C., i.e., a low oxidation rate, excellent spalling resistance of the formed oxide layer, high electrical conductivity, and a small difference in thermal expansion from an electrolyte.